The Motion Picture Experts Group (MPEG) defines MPEG-1 and MPEG-2 standards for achieving compatibility among several image compression and decompression techniques. In MPEG-2, for example, address scanning techniques include scanning the result of a discrete cosine transform (DCT) by alternate address scan and zigzag address scan. A publication by MPEG entitled "Generic Coding of Moving Pictures and Associates Audio Information: Video" is hereby incorporated by reference in its entirety to provide detailed information about the MPEG-2 standard. Alternate and zigzag address scans have also been adopted and used in other video coding standards, such as the international telecommunication union-telecommunication standard sector (ITU-T) H.263 entitled "Video Coding for Low Bitrate Communication." The zigzag address scan has further been adopted and used in ITU-T H.261 coding standard entitled "Video CODEC for Audio Visual Services at Px64 kbit/s."
In FIG. 1, there is shown a general flowgraph of the process of compression and decompression of video images. During data transmission, a variable-length decoding circuit 1 compresses coded data containing a digital representation of video signals for the storage media. An inverse scan 2 sequentially arranges the quantization coefficients by an alternate scanning or zigzag scanning process. An inverse quantization 3 multiplies the coded data by an inverse quantization coefficient. The coded data are then multiplied by Inverse Discrete Cosine Transform (IDCT) coefficients in the IDCT 4 to obtain two-dimensional data in spatial domain. The motion compensation 5 generates decoded pels.
FIG. 2A is a prior art diagram of an access pattern in a zigzag scan. In zigzag scanning, the scanning is performed in a zigzag pattern starting from the low-frequency area, in which large coefficient values are concentrated, permitting transmission of large coefficient values intact but omits the zeros occurring in high frequency areas. The progression of the access pattern in the zigzag pattern starts at pixel 0, moving to pixel 1, then moving to pixels 2, 3, 4, and so on as shown in FIG. 2A until pixel 63, at which time the procedure returns to pixel 0 and starts the access pattern again. The zigzag scan pattern as shown in FIG. 2A is a recognized format for the standard implementation of a conventional zigzag address scan. FIG. 2B shows a prior art flowgraph of a traditional access pattern in an alternate pattern under MPEG-2. The progression of the access pattern in the alternate pattern begins at the pixels 0, 1, 2, and 3, then jumps to the pixel 4, and continues to process in accordance to the alternate scan pattern. The alternate pattern as shown in FIG. 2B is a recognized format for the standard implementation of a conventional alternate address scan under MPEG-2.
FIG. 3 is a prior art block diagram for implementing the zigzag address scan. A 6-bit counter 6 increments the value of the address locations of the video images. A look-up table 7 receives the value generated from the 6-bit counter 6 that increments the value of the address. The look-up table 7 retrieves the corresponding address location, and generates a 6-bit scan/inverse scan address. The look-up table requires a large memory capacity to store the 6-bit address locations of the zigzag address scan for MPEG-1 applications, and requires an even larger memory capacity for both zigzag and alternate scans for MPEG-2 applications.
Accordingly, it is desirable to have methods that operates with a more efficient access patterns for alternate and zigzag scans.